The human immunodeficiency virus type 1 (HIV-1) exploits a large number of host cellular proteins in its life cycle. Among the more prominent proteins is the human cyclophilin A (CypA) which is utilized by the virus for optimal infectivity and viral replication. A number of putative functions have been postulated for CypA but our understanding of its precise role within HIV-1 life cycle is still rudimentary. In response to the PA-06-388 announcement, a combined solution NMR and biophysical simulation investigation is proposed in the current R21/R33 application to determine the detailed three dimensional structure of the HIV-1 capsid protein (CA) bound to cyclophilin A. Because of its inherent flexibility, tendency to oligomerize, and its monomer-dimer equilibrium in solution, a structural study of the wild type full-length CA bound to CypA has frustrated attempts by crystallography and NMR spectroscopy. Thus in this investigation we will study the CypA/CA complex using a double mutant CA that exists as a monomer in solution, and retains all the critical properties of the wild type CA including assembly activity. Utilizing 3D/4D-NMR spectroscopy, residual dipolar coupling measurements and biophysical simulations, we will determine the detailed solution structure of the double mutant CA, and of its complex with human CypA. A knowledge of the detailed structure of the CypA/CA complex will provide a structural biology basis in enhancing our understanding of the precise role of CypA within the life cycle of the virus. Such a structure is also likely to contribute to the development of new inhibitors of CypA-CA interaction with therapeutic potential. PUBLIC HEALTH RELEVANCE: This investigation is aimed at the determination of the detailed three dimensional structure of the human Cyclophilin A bound to a mutant capsid protein of HIV-1 virus using a combined solution NMR and biophysical simulation approach.